1. Field of the Art
This invention relates to flexible or soft thermoplastic block copolymers of propylene having excellent heat resistance and impact resistance. More particularly, the invention relates to thermoplastic block copolymers which are resin materials having physical qualities ranging from flexible or soft to semirigid and possessing in balanced combination both heat resistance and impact resistance of excellence not found in resin materials known heretofore.
In recent years, the development of thermoplastic materials of physical natures ranging from flexible to semirigid, which have heat resistance, particularly the retention of shape against outside force at high temperatures, and impact resistance, especially the strength to withstand impact at low temperatures, has been sought with ever-growing expectations in many fields for uses of these materials in products such as motor vehicle parts, parts of home electrical appliances, coatings of electric cables, tubes and pipes, and various containers.
Typical flexible materials are polyvinyl chlorides containing plasticizers, ethylene-vinyl acetate copolymers, and ethylene-.alpha.-olefin copolymers. While these materials have excellent impact resistance at low temperatures, they are deficient in resisting deformation when heated. In contrast, random copolymers of propylene and another olefin, in general, can become flexible materials having deformation resistant properties at high temperatures that are superior to those of the above mentioned copolymers and also have excellent impact resistance at room temperature but are weak in withstanding impact at low temperatures.
2. Prior Art
In the prior art, much effort has been directed toward the improvement of impact resistance of propylene-based polymer materials. Representative of such effort are those relating to, for example: random copolymers of propylene and ethylene (as disclosed in Japanese Patent Publication Nos. 22052/1967, 42988/1971, etc.); block copolymers of the propylene and ethylene (as disclosed in the specifications of Japanese Patent Publication Nos. 14834/1963, 21494/1963, 1836/1964, and 22688/1967, U.S. Pat. No. 3,624,184, and British Pat. Nos. 889,230, 957,777, and 1,134,660, etc.); random copolymers of propylene and butene-1 (as disclosed in the specifications of U.S. Pat. Nos. 1,918,457, 3,278,504, and 3,332,921 and Japanese Patent Laid-Open Publication Nos. 38787/1975, 79984/1978, etc.); random copolymers of propylene and hexene-1 (as disclosed in Japanese Patent Laid-Open Publication No. 53983/1974, etc.); and random copolymers of propylene and 4-methyl-1-pentene (as disclosed in Japanese Patent Laid-Open Publication No. 104686/1978).
These contributions to the prior art have certainly succeeded in varying degrees on the point of obtaining propylene-based polymers of improved impact resistance. However, these polymers are rigid or semirigid, or those which are flexible are not amply improved in impact resistance at low temperatures or have poor heat resistance. Thus, none of these polymers fully satisfies all of the requisites of the polymers which this invention seeks to provide.
We have previously proposed, as a propylene-based flexible thermoplastic resin material having excellent heat resistance and transparency, a block copolymer comprising propylene and a straight-chain .alpha.-olefin having 5 to 12 carbon atoms (as disclosed in Japanese Pat. Appln. No. 68532/1979, and U.S. patent application Ser. No. 152,605) now U.S. Pat. No. 4,308,361. This block copolymer is considered to have a considerably improved impact resistance but cannot fully meet the requirement for impact resistance of a high degree.